Mechanism of Fatigue Crack Growth at Polymer/metal Interface
Zhang, Zhehua
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Permalink
https://hdl.handle.net/2142/82870
Description
Title
Mechanism of Fatigue Crack Growth at Polymer/metal Interface
Author(s)
Zhang, Zhehua
Issue Date
1997
Doctoral Committee Chair(s)
Shang, Jian Ku
Department of Study
Materials Science and Engineering
Discipline
Materials Science and Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Applied Mechanics
Language
eng
Abstract
Fatigue crack growth along an Al/epoxy interface was examined under mixed-mode conditions. To provide different combinations of Mode-I and Mode-II conditions, a flexural peel (FP) technique was developed, which proved to be particularly useful for subcritical crack growth studies. Using this technique, fatigue crack growth resistance of the interface was found to increase with the mode II component and was expressed as a function of the $\rm G\sb{II}/G\sb{I}$ ratio. Combined with the study of the epoxy thickness effect, the contribution of the plastic dissipation at the crack tip to the mode effect was identified using finite element analysis. This shear-enhanced crack growth resistance of the interface was further verified through the investigation of the asperity shielding by artificially controlling the surface roughness condition using chemical surface treatments. The dependence of mode-mix resulted in a gradual transition of the crack growth path from interfacial, at the low $\rm G\sb{II}/G\sb{I}$ ratio, to cohesive at the high $\rm G\sb{II}/G\sb{I}$ ratio. This effect of crack path selection on crack growth resistance was further explored in the study of fatigue crack growth in double cantilever beam (DCB) specimens where a high fatigue strength was related to a fluctuating fatigue crack growth path. Associated with crack growth path selection as governed by the mix-mode condition, the failure mechanisms in the polymer were different as seen in the microscopic characterizations. Under predominantly Mode-I loading, the damage in the polymer took the form of crazing and cavitation. In contrast, laminar shear occurred under high shear loading, resulting in a larger amount of plastic dissipation at the crack tip and improved fatigue crack growth resistance. Based on the understanding of the mixed-mode fatigue crack growth, a life prediction method for the bi-material interface was proposed.
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